The application of dry powder coating to substrates has been utilized for many years. A way of applying dry powder is the utilization of electrostatic powder spraying technique. The electrostatic powder spray coating method comprises a combination of elements, namely a way of delivering the powder from a suitable supply to a position adjacent the article surface; a way of charging that powder to an electrical sign opposite to the surface so it will be attracted to it; a way of adhering the deposited particles to the surface; a way of fusing this powdered material so that it can flow into a suitable coating; and a way of accumulating or collecting the powder that escapes deposition so it will not become a contaminant but will be fit for reuse in subsequent coating operations.
In electrostatic powder spraying, charged powder in an air stream is directed toward the object to be coated so as to bring the powder adjacent the surface. Deposition generally results after the object has been exposed to the powder spray for a relatively short time. Manipulation of the part or the spray device exposes the various surfaces to the powder.
Alternative to an electrostatic technique is the application of powder in an air system. In the application of powdered plastic materials or powdered coating materials, a process similar to that used for dry enameling has come into use for forming plastic coatings. Basically, one heats the surface to be coated at a temperature above the melting point of the powder and while the substrate is at this temperature, distributing the powder over the surface. When the powder hits the surface, a portion of it will melt and adhere. Added powder will adhere with continuing application as the collected powder layer melts and becomes tacky. The part can then be post heated, if such is required, to flow or cure the powder layer.
See "Powder Coating"by Emery P. Miller et al., Society of Manufacturing Engineers, 1974, pp. 1-36.
In the application of powder to a substrate in particular by electrostatic powder spraying technique, the objective is to spread an air powder mixture into an even cone shape pattern without reducing the discharge of the powder stream and the effectiveness of the electrode. In addition, it is particularly helpful to be able to powder coat difficult to reach parts without the use of expensive masking such as stickers and tapes, which need to be applied by hand. Also, it would be particularly helpful in order to complete a coating of a part without having to dip the part in a separate liquid coating composition. Such treatments likewise require pollution control abatement systems to reduce emissions and require more maintenance.
It is particularly desirable to be able to have a powder coating application that would spread the air powder mixture in an even pattern onto he substrate.
It is an object of the present invention to apply the powder to he substrate in a swirling effect as it exits the tip of the spray gun.